Nutrition/Metabolism Part 2

Energy Expenditure

• basal metabolism- involuntary processes required for life
  • makes up 60-75% of energy expenditure
  • includes breathing, circulation, regulating body temperature, making tissues, waste removal, nerve signals
• basal metabolic rate (BMR)- rate of energy expenditure under energy conditions
  • influenced by weight, gender, age, exercise, hormones, fasting
• lean body mass (muscle) has a higher BMR than fat body mass
  • as you age, lean body mass decreases → so BMR decreases too
  • men tend to have higher BMR than women because they generally have more lean body mass
• total energy expenditure = BMR + energy used in digestion/absorption + activity level

Energy Balance

• energy balance- when energy consumption equals energy expenditure
  • results in weight maintenance
• negative energy balance- when energy intake is less than energy expenditure
  • results in weight loss
• positive energy balance- when energy intake is more than energy expenditure
  • results in weight gain

Protein Metabolism

• Protein is an essential nutrient
• Proteins are made of 20 different amino acids
  • There are 9 essential amino acids that cannot be made by the body and must be obtained from the diet
• When we eat protein, that gets broken down into amino acids that circulate in the bloodstream.
• The amino acids then go inside cells to synthesize our own proteins
  • The proteins we synthesize can last for varying amounts of time
  • proteins like hemoglobin or collagen in the bones can last for a long time
  • regulatory proteins may only be around for a little while before they are degraded
  • the longevity of a protein can be expressed by its half-life- the amount of time it takes for 50% of the protein sample to be degraded
• We can also use proteins to synthesize other nitrogen-containing compounds such as the nitrogenous bases of DNA
• If our protein intake fulfills the needs of replenishing the amino acid pool and synthesizing nitrogen-containing compounds, the amino acids get broken down into ammonia and the carbon skeleton
  • ammonia is highly toxic and is made into urea in the liver and kidneys
  • urea is eliminated in the urine
  • the carbon skeletons get stored as fat in the fed state
  • in the fasting state, the carbon skeletons can be used in 2 ways
  • the carbon skeletons of ketogenic amino acids can be modified into a compound that directly is used in energy production
  • the carbon skeletons of glucogenic amino acids can be modified into glucose

Carbohydrates

• 2 main types- sugars (mono or disaccharides) and fiber (usually plant polysaccharides)

• starch and glycogen are glucose polysaccharides

• starch is a glucose polymer where the glucose molecules are joined together in α-1,4 glycosidic linkages (a bond between carbon 1 of one glucose molecule and carbon 4 of another glucose molecule where the OH on carbon 1 points below the ring)

• **glycogen **is a glucose polymer where the glucose molecules are joined together in

  α-1,4 linkages and α-1,6 glycosidic linkages (a bond between carbon 1 of one glucose molecule and carbon 6 of another glucose molecule where the OH on carbon 1 points below the ring)

• cellulose is a glucose polymer where the glucose molecules are joined together in β-1,4 glycosidic linkages (a bond between carbon 1 of one glucose molecule and carbon 6 of another glucose molecule where the OH on carbon 1 points below the ring)

  • cellulose is indigestible in humans because we do not make an enzyme to digest β-1,4 glycosidic linkages
  • cellulose passes through the gut and gives bulk to stool

Fiber and Health

• Fiber is important in the diet because it helps with
  • normalizing bowel movements
  • help maintain bowel health
  • lower risk of hemorrhoids/diverculitis
  • lower risk of colon cancer
  • soluble fiber (fiber that dissolves in water) lowers serum cholesterol
  • fiber can also reduce blood pressure and inflammation
• whole grains control blood sugar levels better than refined carbohydrates
  • help control weight
  • more filling
  • help you eat less
  • longer duration of satiety
• increased fiber intake also reduces risk of death from cardiovascular disease and certain cancers

Why does there need to be glucose in the blood?

Red Blood Cells

• RBCs don’t do the krebs cycle or oxidative phosphorylation because their function is to transport oxygen, so they shouldn’t be using it
  • RBCs have no mitochondria
• RBCs exclusively do anaerobic glycolysis
• basic rundown of glycolysis
  • glucose gets phosphorylated to glucose-6-phosphate (G6P)
  • G6P gets broken down into 2 pyruvates
• 2 pyruvates are then made into 2 lactates so 2 H+ is released to continue glycolysis
• This only nets 2 ATPs per glucose

Brain and Nervous Tissue

• nervous tissue carries out all 3 steps of cellular respiration (glycolysis, Krebs/Citric acid/TCA cycle, and oxidative phosphorylation)
• It is very dependent on glucose and cannot use fatty acids for energy
  • fatty acids do not cross the blood-brain barrier easily
• during a prolonged fast, the brain can adapt to using ketone bodies for energy

Metabolism of Glucose

• insulin is released when blood glucose levels increase → fed state
• glucagon is released when blood glucose levels decrease → fasting state
• fatty acids provide the energy for gluconeogenesis to occur but the carbons themselves come from amino acids

Fats

• fats are stored in the body as triglycerides- a glycerol molecule with 3 fatty acid chains

• the H in the carboxylic acid of the fatty acid and OH of the glycerol are removed to make a bond between glycerol and the fatty acid- ester linkage

• this happens 3 times because glycerol has 3 alcohol groups

  

Lipid Metabolism (Fasted State)

• hormone-sensitive lipase in adipose tissue gets activated by high glucagon & low insulin levels
• lipase breaks triglycerides into 3 fatty acids and glycerol by hydrolysis
• the fatty acids get bound to albumin to be released into the bloodstream

Ketone Body Synthesis & Utilization

• acetoacetate and beta-hydroxybutyrate are called ketone bodies because they have ketone groups in them
• acetoacetate can spontaneously break down into acetone
  • acetone gives off sweet smell in breath/sweat/urine → how HCPs know when pt is in ketoacidosis

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